Meiosis - Biol2 Revision

Introduction to Meiosis

Meiosis is a form of nuclear division that is involved in sexual reproduction in humans.

Why is meiosis necessary? Without meiosis we would divide and produce cells that were genetically identical, and therefore we would all be clones of our parents.

Meiosis produces 4daughter nuclei,each withhalf the number of chromosomes as the parent cell.

What is the DIPLOID number? The diploid number means to have a full set of 46 chromosomes. ALL of your cells in your body, excluding sex cells (sperm/egg) are of the diploid number. Sex cells, or gametes, contain 23 chromosomes and so are HAPLOID, so that offspring are made up of half of their parent cells, e.g. 23 chromosomes from their mother, and 23 chromosomes from their father.

Our 46 chromosomes within our body cells are paired up into homologous pairs. This means you have a MATERNAL chromosome and a PATERNAL chromosome, one from each parent.

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Stages of Meiosis

Meiosis involves TWO nuclear divisions.

In STAGE 1 of meiosis (meiosis 1): the homologous chromosomes pair up and their chromatids wrap around each other (this will be explained later, but is a process called crossing over). By the end of this stage the chromosomes have separated, with one chromosome from each pair going in to one of the TWO daughter cells. This means each daughter cell has 46 chromosomes, because the original combination of the homologous pairs had 92 chromosomes, and has now been divided into two.

.. Of course, because GAMETES have to have 23 chromosomes, the cells have to divide again. This is MEIOSIS 2: the chromatids of each chromosome move apart. At the end of meiosis 2, 4 cells have been formed, each with 23 chromosomes. These are the gametes, the sex cells, and so in women they are eggs, and in men they are sperm.

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Meiosis producing genetic diversity

Independant Segregation: When homologous pairs 'line up' in meiosis one, they do so at random. This means of of each pair will pass on to the daughter cell, and the chromosomes of the daughter cell are therefore random.

Genetic Recombination: When the homologous pairs line up alongside each other, the chromatids break twist and equivalent portions of the chromosome are broken off and exchanged to the homologous partner. This creates new combinations of DNA and therefore new genes.